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October 10, 2008
PHY2054 DiscussionFall ‘08
Quiz 5 (Chapter 17.518.7)
Name:
UFID:
*1. (5pts) Suppose that you wish to fabricate a uniform wire out of 2.00 g of copper. If the wire is to
have a resistance of 0.300
Ω
, and if all of the copper is to be used, what will be the length of the
wire? The resistivity and density of copper are 1.70
×
10
8
Ω
m and 8.92
×
10
3
kg/m
3
.
The volume of the copper is
V = m/density = (2×10
3
)/(8.92×10
3
) = 2.24×10
7
m
3
Since volume = cross sectional area × length, we can express resistance in terms of length and
volume:
R = ρL/A = ρL/(V/L) = ρL
2
/V
L = √(RV/ρ) = √[0.3×2.24×10
⇒
7
/(1.70×10
8
)] = 1.99 m
*2. (5pts) The heating coil of a hot water heater has a resistance of 8.00
Ω
and it operates at 120 V.
Assuming that the water absorbs all of the energy converted by the heating coil, calculate how long
it takes to raise the temperature of 1.50 kg of water from 20.0
˚
C to the boiling point? (Specific heat
of water:
C
= 4.187
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 Fall '08
 Avery
 Physics, Resistance

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